High-purity metals are desired for many modem processes, such as, for example, as solders, sputtering targets, and applications in semiconductor devices. For instance, high purity cobalt can be desired for formation of sputtering targets. In particular applications, a film of cobalt is sputter-deposited from a high-purity target, and onto a silicon substrate. The film is then subjected to a heat treatment to form cobalt disilicide (CoSi2). Cobalt disilicide has low resistivity and low formation temperature, and is considered a good alternative to titanium disilicide (TiSi2) in integrated circuit applications. It is thus possible that cobalt will partly replace titanium in the manufacture of new generation chips. Cobalt sputtering techniques can also be applied to the manufacture of data storage devices, flat panels and other similar products. Considering the rapid development of the electronics industry, it is believed that a potential market exists for cobalt targets of a purity of 4N or greater.
Cobalt is recovered as a co-product of copper in Central Africa, and as a by-product of hydrometallurgical refining of nickel elsewhere. In the African plants, copper-cobalt concentrates are roasted and leached in a sulfuric acid solution. Copper and cobalt are recovered separately from the leach solution by direct electrowinning. For hydrometallurgical refining of nickel, a variety of techniques such as selective precipitation and crystallization, solvent extraction and ion exchange, are used to separate cobalt from nickel. Cobalt is then electrowon from sulfate or chloride solutions. In addition to the electrowinning process, cobalt can also be produced as metal powder using a soluble cobaltic amine process. Nickel, as a sister element to cobalt, is always found in cobalt produced by these processes. Other impurities in the resulting cobalt include alkali metals (such as Na, K), radioactive elements (such as U, Th), transition metals (such as Ti, Cr, Cu, Fe) and gaseous impurities (with gaseous impurities being those measured by LECO, and being O, C, S, N, H).
Nickel is not easily removed from cobalt. This is because of the similarity of cobalt and nickel in a series of properties. Cobalt and nickel can form thermodynamically ideal liquid and solid solutions. The solidification of a Co—Ni system takes place in a temperature interval of only a few degrees. The standard electrode potentials of the reactionsCo2+2e−→Co;andNi2++2e−→Niin aqueous solutions at 25° C. are −0.28V and −0.23V, respectively. The difference of both potentials is only 0.05V. All of these factors make the separation of cobalt and nickel very difficult.
For the semiconductor industry, it can be important to minimize impurities in cobalt sputtering targets in order to prevent problems with semiconductor chips comprising sputter-deposited cobalt. Specifically, alkali metals (such as Na and K), non-metallics (such as S and C), and metallics (such as P within the context of this document) are undesirable because these elements are considered to be very mobile and may migrate from one semiconductor device layer to another. Fe is another element that can be undesirable. Specifically, Fe can affect the magnetic properties of a material, which causes concern for magnetic inconsistency. Further, Fe, as well as Ti, Cr, Cu can be undesirable in that they can cause problems with connections at semiconductor device interfaces. Additionally, gaseous impurities (such as oxygen) are undesirable since they can increase electrical resistivity of the cobalt and the cobalt silicide layer in semiconductor devices. Increasing O levels also increase particulates that form during application of metallization layers. These particulates can degrade or destroy a cobalt silicide layer. Ni impurities in cobalt are undesired since Ni can influence the pass-through flux of cobalt sputtering targets. And finally, radioactive elements such as U and Th are undesirable in Co since they emit alpha radiation, which can cause semiconductor device failures.
Other metals, besides cobalt, also have applications as high-purity materials (for instance as sputtering targets or as solders), and it would be desirable to develop purification methods which can be applied not only to cobalt, but also to other metals.